High-speed data communications systems need to support the aggregate bandwidth requirements of current and future applications such as telecommunication technology, supercomputer interconnection, high-quality video conferencing and multimedia traffic. There is a general consensus that these bandwidth requirements can most easily be attained by using optical transmission technologies. Dense optical wavelength division multiplexing (WDM) appears to be the hardware backbone for such networks. Dense optical WDM is a method of multiplexing a large number of optical data channels on a wavelength basis (e.g., each divided wavelength region is regarded as a different channel and is routed and manipulated separately from all other divided wavelength regions).
Dense WDM requires advanced optoelectronic components and subsystems capable of handling the extremely high aggregate bit rates and traffic levels demanded by modern optical data communications systems. One very critical component needed for the implementation of WDM packet-switched systems is a tunable filter. A tunable filter is used to extract one or a subset of optical channels from the multitude of optical channels entering the filter. The subset of extracted channels can be varied in time by dynamically modifying the filter configuration
Some common optical filters are based on classical interferometers, and include Fabry-Perot and Bragg filters. Such filters are tuned by mechanically or thermally moving the resonating structure, and the tuning speed is therefore comparatively slow, typically of the order of milliseconds.
Another type of tunable filter is based on the acousto-optic effect. Such components depend on the interaction between an acoustic wave generated in the device, and the optical signal input into the filter. The tuning is achieved by altering the frequency of the acoustic wave, which can be accomplished by altering the frequency of the electronic signal used to generate the acoustic wave. These filters are, however, polarization dependent, which may require correction and cause various other practical problems.